Means and method for measuring direct capacities



sept. 8, 1925. www@ G. A. CAMPBELL MEANS AND METHOD FOR MEASURING DIRECT CPACITIES `Filed July 14. 1923 2 Sheets-Sheec l A T TORNE Y Sept. w25 1,552,465

G. A. CAMPBELL MEANS AND METHOD FDR MEASURING DIRECT CAPACITIES Filed July 14. 1923 2 sheets-sheet 2 INVENTOR 6; f @1M/Dd A T TORNE Y least assumed.

Patented Sept. 8, 1925.

ITED STATE GEORG-E A. CAMPBELL, OF UPPER MONTCLAIB, NEW JERSEY, ASSIGNOR TO AMERICAN l TELEPHONE AND TELEGBAPH COMPANY, A CORPORATION 0F NEW YORK.

MEANS AND METHOD FOR MEASURING DIRECT CAPACITIES Application led July 14, 1923. Serial No. 651,587.

To all whom t may concern.'

Be it known that I, GEORGE A. CAMPBELL, residing at Upper Montclair, in the county of Essex and Sta-te of New Jersey, have invented certain Improvements in Means and Methods for Measuring VDirect Capacities, of which the following is a specification.

This invention. relates to the measurement of capacity and particularly vto means and methods for the measurement of direct capacities.

Formerly it was the practice among telephone engineers to make so-called fmutual capacity measurements between the `conductors comprising one or more telephone circuits. Mutual 'capacity measurements, however, are of little value in connection with cross-talk determinations and consequently it'has been more recently the practice to measure the so-called direct capacity thereby permitting accurate controloflcross-talk and determiningv more. completely how telephone circuits behave under all possible connections. By the direct capacities of an electrical system, is meant the capacities of a particular direct network which is equivalent to the given electrical system as wflll be more specifically 'set forthA hereina ter.

Telephone cables. and other parts of th telephone plant present the problem of measuring capacities which are quite vimpossible to isolate but which must be meas-y ured just as they occur in association with other capacities; and these associated capacities may be much larger than the particular direct capacity which is necessary to accurately measure, and have admittances yoverwl'ielmingly larger than the direct conductance which is often the most im ortant quantity. This renders it desira le to measure direct capacities as distinguished from ordinary capacity measurements where isolation of the capacity is secured or at Various methods and arrangements for making direct capacity measurements have been proposed,'of which the one in most common use is what is known as the substitution alternating-current bridge method. T his method, however, involves two settings of the bridge for each individual direct capacity measurement, as the capacity to -be measured must first-beronnected across one arm et the bridge and the bridge then balanced after which the capacity is connected across another arm of the bridge and a bal-- a given direct capacity maybe made from a single settingV of the bridge.

The invention may now be more fully understood from the following description thereof when read in connection with the accompanying drawing in which Figure 1 illustrates how simple fundamental capacity systems may be resolved into equivalent direct capacitv networks.

Fig. 2 illustrates one circuit arrangement for measuring direct capacities in accordance with .the present invention.

Fig. 3 is a simplified diagram showing the distributionv of thev capacities in the circuit of. Fig. 2.

Fig. 4 is a s1mplified diagram showing how the negative resistanc'e of the Y network of Fig. 2 may be obtained by using a vacuum tube.

Fig. 5 illustrates schematically a form of shielded condenser which may be used in connection with the circuit of the present invention.

Fig. 6 is a circuit arrangement illustrating how the method may be applied to the measurement of the direct capacity between two terminals of a network involving more than three terminals.

F ig. 7 illustrates how a complex network involving more than three external terminals may be resolved into an equivalent direct network.

Fig. 8 is a simplified diagram showing the distribution of capacities in the circuit arrangement of Fig. 6.

Fig. 9 is a circuit arrangement illustratingv another form in which the `invention may be.-applied. v

Fig. 10 is a simplified circuit illustrating 1the capacity distribution inthe circuit of ido il@ y fact that twc condensers of capacities 'CL terminals, which is equivalent to two capacities connecting these terminals to a concealed branch-point, -is equal to the product of the two capacities divided by the total capacity terminating at the concealed branch-point, i. e., its grounded capacity. These rules may be used to determine the direct capacities of any network of condensers, with any number ot accessible terminals and any number of concealed branchpoints. Thus, all concealed branch-points may be initially considered to be accessible, and they are then eliminated one after another by applying these two rules; the final result is independent of the order in which the points are taken; a network ot' capacities, directly connecting the accessible terminals, without concealed branch-points or capacities in parallel, is the iinal result.

Fig. l shows two elementary cases of direct capacities and also is an illustration of a more complicated system of capacities with three terminals 1, 2 and 3 assumed to be accessible and the fourth terminal inaccessible or concealed. Thus we see that the direct capacity between the terminals l andA I 2, of the direct network shown at the lower central part of the ligure, and which is equivalentl to the more complicated network of capacities shown at the left, has a value where G4, the grounded capacity of the terminal 4, equals C14-i-C24-i-C34.

Generalizing, we have the following definition:

The direct capacities of an electrical sys- -tem with n given accessible terminals are physical substitute for the given electrical system for all purposes. The 'following statements of the additive property of di- Vrectccapacities show the simple manner in which direct capacities are altered under some of the most'important externalA operations which can be made with an electrical network: Connecting a capacity between two terminals adds that capacity to the direct capacity betweenthese terminals, and leaves all other direct capacities unchanged. Connecting the terminals of two distinct electrical systems, in pairs, gives a system in which each direct capacity is the sum of the corresponding two direct capacities in the individual systems. Joining two terminals of a single electrical system to form a single terminal adds together the two direct capacities from the two mergedterminals to any third terminal, and leaves all other direct capacities unchanged, with the exception of the direct capacity between the two merged terminals, whichbecomes a short circuit. Combining the terminals into any number -of merged groups leaves. the total direct capacity between any pair ot groups unchanged, and short-circuits ally direct capacities within each groups` lVith this' understanding of the Aproperties ot direct capacities in mind, thepresent in-` vention will now be explained in detail.

Referring to Fig. 2 which shows an'arrangement for determining direct capacities by a single bridge setting, it will be observed that instead ot thc usual form ot' lVheatstone bridge balance a Y network is connected to three corners a, b and c of a bridge. The Y network is made up of two resistances R, R and a negative resistance -lt The Y network has a null-impedv ance between the corner Z) and the corners `a and c connected together. This will be apparent from the tact that the resistance of the two lower arms in parallel is which, taken in series with the negative rei sistance of the upper arm, gives a zero resistance tor the network. The total conductance connecting the branch-point of the Y tothe three bridge corners a, b and c, is 0.y The conductance between any one corner and the remaining two corners joined together, is infinite or in other words, the Y acts as a short circuit under all these conditions. On the other hand, it' corner a, b or c 'is left floating and ignored the conductance between the other two corners is Q/R, l/QR or Q/R, respectively, and the Y is not a short circuit. These statements are verified at once by applying the familiar expressions for rcsistances in parallel and in series.

The three terminals l, 2 and 3 ot' the network Whose capacity is to be measured are connected to corners (l, 0 and b, respectively, of the balance as indicated in Fig. 2. The direct capacity Cn between terminals 1 and lla',

. denser are associated. with terminals 7I and 8.

lil

2 is the direct capacity to be measured. ln order to balance the bridge an .adjustable capacity C" is connectedbetween corners a and d. This. capacity may be of any known adjustable type, but as it is undesirable that there should be any leakage between the main terminals 7 and 8 of the capacity C', a leakage terminal 9 is provided, this leakage terminal being connected tothe corner b of the bridge. As an example of how the leakage terminal may be associated with the condenser, a form of construction is illustrated in Fig. 5. Here the base plate 9, supporting the leaves of the condenser, constitutesthe leakage terminal. lThe plates ci the conwhich are insulated from the leakage ter.-

minal 9 in a well known manner. By this.'

construction leakages either overor through the dielectric supports from either -of the two main terminals 7 and 8 rnustv passto the leakage terminal 9. rIhere can be no leakage directly from one ofthe main ter'- ,3 ot' the network is connected between corners b and c and as these corners are at the same potential, it is apparent that this direct capacity will not enter into the balance obtained by adjusting the condenser C .i The third direct capacity C13 is connected between corners `b and (Z and consequently is in veliect shunted across the high irequency source. lt does not, therefore, enter' into the balance. Consequently, when the condenser C is adjusted until the telephone T1 minals through the support to the other.l

Consequently, there is no phase angle defect' in the standard capacity due to leakage' and that due to dielectric hysteresis in the insulating materialV is reduced to anegligible amount by extending the leakage plates .beyond the dielectric Jso as to ,intersect practically all lines of lnductance passing through any support.

Vhile various known forms of .negative resistance are available for the negative resistance leg of the Y, it is apparent that an electron tube'provided with'a feed-back connection may be used for this purpose with considerable advantage; The connections for the electron tube are illustrated in Fig. 4. It is well known that an electron tube provided with a 'feed-back circuit of this type maybe given a definite negative resistance characteristic at a given frequency and as the current supplied to the bridge from the source 10 is an alternating current of a definite frequency, it is evident that the negative resistance c ara'cteristic is readily controllable. Correct adjustment of. the negative resistance may be checked by ob- `servingwhether' there is silence in a 'tele'- phone T, after the bridge has been balanced with respect to the telephone T1. It is, ot course, obvious that the telephone T, is connected across neutral points of the bridge when the bridge is balanced. It the negative resistance has the right value, corners .0., b arid c will al1 three bel at the saine potenquency.

- tial so that there should also be silence in' l the tele hone T2 which, isconnected between points and 'cf Where the negative resis-` tance-fused is one of a type which maintains 'its characteristic constant, thetest involving the telephone T2 need be made'only when the bridge 1s set 'up or. there is a change' in frebalance.

equivalent to the leakage terminal of` 'is silent, the setting of the condenser will.

With vregard give the directl capacity C12.

' the adjustableit should be noted that the leakage ycapacity C79 between terminals Y and 9 is connected to corners a and b of the bridge. and asfthese two corners are at the same potential, this leakage does not enter into the So also the leakage capacit C89 between terminals 8 and 9 of the con enser C is connected between corners b and cl ot capacity c,

. the-bridge and in ed'ect shunted across the source so that this capacity likewise does not a'ect the balance. Consequently, the direct capacity C12 which-is to be measured,

l (lll is balanced direct-ly by the capacity 0,8 be- Y tween the main terminals of the adjustable condenser C andthe capacity to be measured may therefore be read directly from the setting of the balancing condenser.

On account of the usual behavior of the Y network even when taken alone, it may not immediatelybe apparent how it will aiect the 'operation of the bridge of Fig. 2 to connect direct ca acities between the corners a, b and b, c. *or this reason it is highly desirable to find an equivalent network, the behavior of which 1s more readily comprehended. For this purpose we may make use of a Y networkwhich is* to the original Y and the direct capacities Cab and Cb., taken together, these capacities being, in the case actually illus- .trated in Fig. 3, capacities C, and C23, respectiuely.' Y

It is wellknown that any admittance delta may be replaced by a star having admittances equal to the sum ot' the products of the delta admittances take-n in pairs di- `vided by the cppnsite delta admittance. Let us consider,y for exam le, the delta arrangement shown'l at the lbwer left in Fig. l, which is made up of a Y comprising capacities C, C2, and C3, connecting external terminals 1, 2 and 3 with capacities C13, C23

llO

p pacities v where and C12 directly lconnecting the terminals. AS. already pointed out, this network is equivalent to the ,simpler direct capacity delta arrangement, having the direct ca- 024634` Gr4 i Cia-iand y 4 connecting the terminals. l, 2 and 3. as Shown.

that the admittance of each element of the equivalent star is equal to the sinn of the products of the delta admittances taken in pairs divided by the opposite delta admittance. or example, the element C1, of the star is given in terms of the elements of the delta by the following. equation:

'ln a similar manner, the values of the other elements of the star lmay be derived so that the three legs of the star may be expressed as follows:

W14: c aca +de, c

.Clim :Cacia G4015 (3) By substituting the'value of G4 in the preceding equation, it will be observed that it is the sum of sixteen terms, each of which is the product oli-three capacities, every combination of three capacities being included; except the four cases in which the three capacities wouldv form a closed circuit.l

' While the preceding formulae are derived upon the .bases of treating the elements of the several networks as simple capacities, it

is obvious that the same .form of expression will hold for more complex quantities than capacities by simply substituting the complex expression for the various capacity terms. Viewed in this light the formulae apply to any admittances, either capacities, conductances, or combinations of the two.

As already stated, the total'conductance connecting'the concealed branch-point of the Y network intFig. 2 is 0. Assuming that the same thing holds for the delta network shown at the lower left in Fig. l., the

sum of the capacities connected to the point 4 will be 0 so that the factor Gr4 lin equations 3 and 4 become 0, hence for this special I case we may obtain from formul 3, the

following.: y p

@Laarne/sain' (5, C14 024i* *014024034 From equation 5, it is seen at once that the arms of the star or Y shown at the lower right in Fig. 1, present the same ratios as the arms of the Y of the equivalent network t'o the extreme lower left in Fig. l. It

follows, therefore, that the direct capacitiesl Cab, -Cbc of Fig. 3 have no effect on the `bridge ratio, `thus the constancy of the bridge ratio holds for all null-impedance bridges regardless 'of the ratio of the arms of the bridge and of the nature of the direct admittance from corners a and c to In attac-ee I short, the. bridge may, be'given any ratio Zl/Z2 by employing a Y network made up of impedances Z1, Z2 and-Z1Z2/(Z,+Z2).

3 Si 0140140@ l 614014 (C13 @1.4024034 l v CACMCM also since we have 'i O23) l- CuCaiCia 0340114623 Cia-lCzs C13 Cas By substituting the values given byv the equations Guanti 8 in equation 5 we have Cnn Clin; Gysi 10 consequently Applying this to the circuit of'Fig. which is possible since the bridge ratio is unity, We find that the three arms ot theI 2o and 26, where R is the original resistance in the Y and C is one-half. the sum of the two actual direct capacities troni?) to a and. l5 to c. The equivalent bridge thus obtained has ratio arms made up of ordinary resistances and capacities and therefore Fig. 4e

used as a bridge can present no unexpected characteristics. The negative rsistances and capacities of the equivalent Y merely atleet the current supplied to the bridge.

Fig. (i shows how a bridge having a Y network of the type shown in Fig. 2 may be used to obtain the direct capacity between two conductors of a larger group than three. ln this case, a group of four conductors is assumed, numbered 1, 2, 3 and 5, conductor 1 being connected to corner d, conductor 2 `to corner c, conductor 3 to corner band conductor 5 to corner b.

rlhe capacities between each of thevfonr .40 terminals 1, 2, 3 Vand 5 taken in pairs and the capacities between cach of the four terminals and the ground point 4, are shown at the left of Fig. 7., By applying the principle setiorth in connection with the discussion of Fig. 1, the complex network at the lett of Fig. 7 may be replaced by a f simple direct network'of six direct capacitics as indicated at the right of Fig. 7. y

Referring now to Fig. 8, which shows the distribution of the equivalent direct capacitieswith reference to' the bridge of Fig. 7, it will be' seen that the direct capaci- :cai-ieieea (lo.

ities ,0'28 and (lf2, are connected in parallel between corners b and c, while direct capacitiesfC'lgand CU, are connected between terminals and d, thereby being ineect shunted across the Source. Direct capacity -Clz which is to be measured is connected between terminals c and i as before. rlhe remaining direct capacity CSr, is short-circuitedby reason ofthe fact that conductors lSand 5 are both connected to terminal consequently this direct capacitydoes not enter into the balance. The ca acities. associated with the balancing con enser C are distributed in the Asaine manner as in Fig. 3. `It will be seen from the diagram of Fig. 8 that'when the bridge is balanced the capacity CJz to be measured will be'equal to the capacity C18 to which. the balancing condenser C .is set.

An ideall transformer, if such a device existed, might replace the Y network of Fig. 2, for example, for the transformer would maintain a constant ratio between the currents in the two windings and act as a shortcircuit when thembridge is balanced. ln a paper by G. A. Campbell entitled. 6Cisoidal oscillations appearing in the transactions of the American Institute of Electrical Engineers f0r1911 at page 873, it is shown that a given Y network may be replaced by an equivalent transformer. For example, reerrin to Fig. 11, if we have terminals 1, 2 and 3 l onnected by a Y network whose arms have admittances Yu, Y2, and Yg, respectively., the network may be replaced by a transformer having. a primary impedance Zn, a secondary impedance ZS and a mutual impedance Zines indicated at the left of the figure. (Se'eFigures 2 and l of Table I oppositel page 884 'of the above mentioned paper in. the transactions of the American Institute of Electrical Engineers.)

Let us determine the error when an actual transformer with impedances Zp, Zs and Zl,s

formulae and by substituting is employed, and capacities C13 and C23 conneotterminals 1, 3 and 2, 3, respectively, as indicated in Fig. 12. The network comprising the transformer and its associated capacities Vmay be re laced by a Y network associated with simi ar capacities as. shown at the right thereof in Fig. 12. This network in turn may be replaced by an equivalent direct net-work consisting of a simple Y as'shown at the extreme right. The ratio of the two lower arms Y 14 and Yf"2 of the 'that the admittances of the three arms of a Y network, such as shown at the right of Fig. 12, may be expressed in terms of the primary, secondary and mutual impedances of the equivalent transformer, as follows:

' "Zvef Zs y 1 l Yfm 12) 34 ZPS Similarly, Y4 which is the algebraic sum of the three legs of the Y joined at point 4, may from equation 12 be expressed as follows: v l

ZZm-Zl Y* TAZMW) (ma.) Z... (13) Substituting they values given by equations 12 and 13 iii-equation 11, the actual rat-io of the bridge arms at the right of Fig. 12 becomes: c

rl `his equation shows that the ratio is not entirelyindependent of the admittances of the capacities C13 and C23 of the transformer network shown at the left in Fig.12,but by suitable design of the transformer, the factor ZD Z,-Z2S may be made to approach 0, so

that the error introduced by connecting the capacities across the corners, 1, 3 and 2, 3 may be made relatively small. For an ideal transformer, of course, this factor would be O. The terms which involve this factorin the numerator and denominator. of equation 14 may be caused to completely vanish by including in series with the two windings of the transformer a properly adjusted negative resistance. A condenser mayalso be included to neutralize the reactance. In this manner anordinary transformer may be made to give the same perfect results as arey obtained with the resistances of Fig.' 2,

a for example.

The manner in which these negative resistances may be lconnected in a circuit involving a transformer, is illustrated in Fig. 9 in which negative resistance Rt is included in series with the impedances ZD, ZS of the transformer windings. j This negative resistance may be of any known type such for example, as a negative resistance obtained from a vacuum tube circuit such as illustrated in Fig. 4. The circuit arrangement of Fig. 9 presents the advantage of calling on the vacuum tube for only a relatively small compensating effect, the greater art of the compensation being accomplished) by means of the mutual im edanceof they transformer. Al condenser t is shown in series with the negative resistance for neutralizing the reactance of the transformer. The connections of the capacities to be measured and of the balancing capacity in Fig. 9 arethe same as in Fig. 2 and the resultant capacity f i distribution isnillustrated in Fig, 10. lt will be obvious from` Fig. lOthat the capacity 0,8 resulting from the adjustment of the condenser C directly balances the capacity C12 to be measured'so that the value' of the latter may'be obtained directly from the reading of the adjusted condenser C.

lt will be obvious that the general principles herein disclosed may be embodied in Amany other organizations widely different fromv ithose illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is: l

1. A bridge for measuring admittances, said bridge having four accessible terminals, anetwork connecting three of the terminals, said network being so proportioned and designed. that the conductance between any` one ofthe three terminals and the remaining two terminals joined together' is infinite.,

an indicating instrument connected to twoL of ,said three terminals, an alternating-current source connected to the remaining terminal of the three and to the fourth terminal of the bridge, an admittance to be measured connected between the fourth terminal of the bridge and one of said three terminals land a standard admittance connected between said -fouith terminal and another of said three `terminals, said-standard admittance being adjustable to balance the ad-3 mittance to be measured.

said bridge having four accessible terminals,l a network connecting three of the terminals, said network being so constructed and proportioned as to produce equal potentials at all three of said terminals, an Vindicating instrument connected to two of said three terminals, an alternating-current source connected to the remainingterminal of the three and to the fourth terminal of the bridge, an admittance to be measured connected between the fourth terminal of the bridge and one of said three terminals and a standard admittance connected between said fourth terminal and another of saidjthree terminals, said standard admittance being adjustable to balance the admittance-to be measured. A bridgeior measuring direct capacities, said bridge having four accessible terminals, a network connecting three of the terminals, said network so proportioned and designed that the conductance between any one of the three terminals and the remaining two terminals joined together is infinite, an indicating instrument connected to two of said three terminals, an alternating-current source connected to the remaining terminal of the three and to the fourth terminal of the bridge, a plural-ity of conductors in capacitative relation to each other, two of said conductors between which a direct capacity exists, bemg connected. the one to y said fourth terminal of the bridge and the other to one of said three terminals of the bridge and the remainder of the Aconductors being connected to another ont` said ,three terminals of the bridge whereby the remaining direct capacities of the system a're so connected as to not Yaffect the balance of the bridge, a standard condenser connected between said fourth terminal of the bridge and another of said three'term'inals, said stand.

ard condenser being adjustable to balance the direct capacity between saidV iirst two conductors. l

4. A bridge for -measuring'direct capacities, said bridge having four accessible terminals, a network connecting three of the terminals, said network being so constructed and propnrtioned as to produce equal potentials at all three of said terminals, an indiinstrument connected to two ofsaid terminals, an alternating-current source connected tothe remaining terminal of the three and to the fourth terminal of (he bridge, a plurality of conductors in capacitative relation. to each other, two of said conductors between which a direct capacity exists, being connected, the one to said fourth terminal of the bridge and the other to one of said three terminals of bridge and the remainder of the conductors being connected to another of said three terminals of the bridge whereby' the remaining direct capacities of the system are so connected as to not affect the balance et the bridge, a

eating three standard condenser connected between said fourth yterminal `'of the bridge and another of said three terminals, said standard ,condenser being adjustable to balance the direct ,capacity between said iirst two conductors.

5. A bridge for measuring adi'nittances,`

' said brid e havin our accessible terminals a Y network interconnecting three of said terminals, said Y network being so designed and proportioned that the total conductance 75 connecting the concealed branch-point of the Y ,toV said three terminals is zero, an alter. nating current source connected rto one of saidfthr'ee terminals and tothe fourth terminal of the bridge, an admittance tdbe measured connecting said fourth .terminali and one of the remaining two of said three terminals and a standard admittance connected between said fourth terminal and the third of said three terminals, said stand- S5 ard admittance being adjustable to balance I `the admittance to be measured.

- so,i proportioned that the conductance connecting the concealed 'branch-point ot the Y to the three ,bridge terminals is zero, an altern ating-current source connected to one of said three terminals and tothe fourth terminal of the. bridge, an admittance to be meas' urcd connecting said fourth terminal and one of the remaining two of said three terminals and a standard admittance connected between said fourth terminal and the third of said three terminals, said standard admittancebeing adjustable to balance :the ad- MV :m-ittance to be measured.

7. A, bridge for measuring direct capacities, said bridge having four accessible terminals, a Y network interconnecting three of said terminals, said Y network being so de-l signed and proportioned that the total conductance connecting the concealed branchpoint of the Y to said three terminals is zero, an alternating current source connected to one ot said three terminals and to the fourth terminal of the bridge, a plurality of conductors arranged in capacitative relation, two of said conductors between which an` equivalent direct capacity to be measured exists, being connected, the one to said fourth terminal and the other to one of said three terminals of the bridge', the remaining of the conductors being connected to another of said three terminals so that the-remaining equivalent direct capacities are soconnected r fai.

8. 'A bridge for measuring direct capaci-'f'-l f combination of positive and negative resistances s'o proportioned that the conductance connecting the concealed branch-point of the Y to the three bridge terminals is zero, an alternating-current source connected to one of said three terminals and tothe fourth terminal of the bridge, a plurality of conductors arranged in capacitative relation, two of said conductors between which an equivalenty direct capacity to be measured exists, beine" connected, the one to said fourth terminal and the other to one of said three-terminals of the bridge, the remainder of the conductors being connected to another of said three terminals so that the remaining equivalent direct capacities are so connected as to not affect the balance of the bridge, a standard capacity connected between said fourth terminal of the bridge and the re,- maining term-inal of said three terminals, said standard capacity being adjustable to balance the direct capacity to he measured.

9,A bridge for measuring direct,l capacities, said bridge comprising four accessible terminals, a Y network interconnecting three ofsaid terminals, said Y network comprising two vequal 'positive resistance legs, and a third negative resistance leg having a resistance equal in value but opposite in sign to the resistance of the other two legs con-- nected in parallel, an alternating-current source connected between the fourth terminal of said bridge and one of said three'terminals, a plurality of conductors arranged in capacitative relation to. each other, two of said conductors between which an equivalen-t direct capacity exists, being connected, the one to said fourth terminal and the other to another of said three terminals, the remainder of the conductors ybeing connected to the first of said three terminals so that the remainingvdirect capacities of the system i do not affect the balance of the bridge and a standard capacity connected between saidv fourth terminal and the third of said three terminals, said standard capacity being adjustable to balance the direct capacity to be measured. .s

10. A bridge for measuring direct capacities, said bridge comprising four acces` sible terminals, a Y network interconnecting three of said terminals, said Y network comprising two equal positive resistance legs, and a third negative resistance leg having a resistance equal in value but opposite in sign to the resistance of the other two legs connected in paralleh'an alternating current source connected between-the fourth terminal of said bridge and one of said three terminals, a plurality of conductors arranged in capacitative relation 'to each saidistandard capacity being adjiistableV to balance the direct capacity to be measured, y

and having a leakage terminal connected to the first of said three terminals.

ll. A bridge for measuring capacities,

vsaid bridge having four accessible' terminals,

a network connecting `three of the terminals,

said network being so proportioned and de- `signed that `the conductors between any one of the three terminals and the remaining two terminals joined together is infinite, an

indicating instrument connected to two of said three terminals, an alternating-current source connected to the remaining terminal of the three and to the fourth terminal of the bridge, a capacity to be measured connected between the fourth terminalA of the bridge and one of said three terminals and a standard capacity connected between the fourth terminal of the bridge and another of said three terminals, said standard capacity being adjustable to the balance of the capacity to be measured and 'having a leakage terminal connected to the third of said three terminals. 1

12. A bridge for measuring capacities, said bridge having four accessible terminals, a network connecting three of the terminals, said network being so constructed and proportioned as to produce equal potentials at all three of said terminals, an indicating instrument connected to two of said three terminals, an alternating-current source connected to the remaining terminal of the three and to the fourth terminal of the bridge, a capacity to be measured connected between the fourth terminal of the bridge and one of said three terminals and a standard capacityconnected between the fourth terminal ofthe 'bridge and another of said three terminals, said standard capacity being adfustable to the balance of the capacity to lie measured and having a leakage terminal connected tothe third of said three terminals. 'l

13. In a bridge comprising four accessible terminals, the method of measuring admittanccs which consists in applying an alternating potential to two of said terminals,

indicating instrument between said remaining two terminals, connecting an admittance 75 nal and the third of said threeterminals,

rseaaes lil. ln a bridge comprising four accessibleA terminals, the method of measuring Jhe capacity between two or a group oi" conductors in capacitative relation which consists in applying a source of alternating potential to two of said terminals, producing a condition of equal potentials at one ,of said two terminals and at the remaining twov terminals of the bridge, connecting an indieating instrument between said remaining' 'two terminals, connecting two of said group of conductors between which the direct capacity to be measured exists, the one to one of said two remaining terminals and the other to one of said rst two terminals conf necting the remaining conductor or'conductors to the other of said irst two terminals, and adjusting; the capacity between one of the terminals to which said source is applied and the other of said remaining` two terminals to balance the direct capacity to be measured.

ln testimony whereof, ly have signed my 30 name to this specification this lth day of July 1923.

enorme a. CAMPBELL. 

